After the whistle blew and prior to the referee assessing the penalty a conference was held by the officials and the too many men call was changed in error. There is only one possible situation in which this could have been deemed a legal line change; even with seven skaters on the ice. Some member(s) of the crew obviously convinced the linesman who stopped the play that this situation applied. They were wrong. Let me explain.
With the play deep in the Washington end zone Bergeron clipped Semin in the head with his stick near the faceoff circle hash marks. The back referee identified the infraction and Washington's sensational young goalie Braden Holtby raced to the bench in favour of an extra attacker as the Caps pushed the puck up ice.
Following along behind Holtby for a line change was defenceman Roman Hamrlik. Holtby arrived at the Capitals bench in advance of Hamrlik and was within the legal five-foot change limit when his replacement jumped onto the ice from the middle of the players' bench. (I believe it was Mike Knuble.) Holtby took his place on the Capitals bench.
It is important to note that if Holtby's replacement (Knuble?) left the bench prior to the five-foot change limit being achieved then Rule 71—premature substitution would have resulted. The linesman would have blown the whistle, the face-off would result at centre ice but no time penalty to the Caps would be assessed. (This would constitute a premature substitution for goalkeeper versus too many men on the ice.)
Holtby was on the bench and his replacement on the ice when Roman Hamrlik arrived near the back door of the Washington players' bench for a change. With Hamrlik still physically on the ice but reasonably within the five-foot change limit, Alex Ovechkin jumped off the end of the bench closest to their attacking zone. All systems were good at this point even with seven players on the ice; legal change for Holtby had been executed and Hamrlik was within five feet of the bench as was Ovi.
The too many men on the ice violation occurred when Alex Semin saw Ovechkin open on the left side and fired a perfect cross-ice pass onto Ovi's tape prior to Hamrlik physically leaving the ice. At this point Rule 74 is applied: "If in the course of making a substitution, either the player entering the game or the player retiring from the ice surface player the puck with his stick, skates or hands or who checks or makes any physical contact with an opposing player while either the player entering the game or the retiring player is actually on the ice, then the infraction "too many men on the ice" will be called."
The linesman stopped play to correctly apply Rule 74 when Ovechkin played the puck with Hamrlik still physically on the ice. The only thing that I can think of is that someone determined that the second man over the boards (Ovi) was intended and deemed to replace Holtby and was able to convince the linesman of it.
No matter what the reason the officials came up with through their on-ice conference, the final call was made incorrectly. Washington was clearly guilty of too many men on the ice and the teams should have played 4-on-4. The announcers got it right — the officials did not.